Most of the power and usefulness of today's digital IC devices can be attributed to the increasing levels of integration. More and more components (resistors, diodes, transistors, and the like) are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal. It takes the shape of a solid cylinder. This crystal is then sawed (like a loaf of bread) to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components are commonly defined photographically through a process known as photolithography. Very fine surface geometries can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex IC's often resemble familiar terrestrial "mountain ranges", with many "hills" and "valleys" as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern, defining the various components, is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using the optical means of the photolithography tool, and is imprinted into the photosensitive layer. To build ever smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer, i.e. optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, the "hills" and "valleys," exaggerate the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (i.e. fully planarized) surface will allow for extremely small depths of focus, and in turn, allow the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) is the preferred method of obtaining full planarization of a wafer. It involves removing a sacrificial layer of dielectric material using mechanical contact between the wafer and a moving polishing pad with chemical assistance from a polishing slurry. Polishing flattens out height differences, since high areas of topography (hills) are removed faster than areas of low topography (valleys). Polishing is the only technique with the capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
FIG. 1A shows a down view of a CMP machine 100 and FIG. 1B shows a side cut away view of the CMP machine 100 taken through line AA. The CMP machine 100 is fed wafers to be polished. The CMP machine 100 picks up the wafers with an arm 101 and places them onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is textured, often with a plurality of predetermined groves 103, to aid the polishing process. The polishing pad 102 rotates on a platen 104, or turn table located beneath the polishing pad 102, at a predetermined speed. A wafer 105 is held in place on the polishing pad 102 and the arm 101 by a carrier ring 112 and a carrier 106. The lower surface of the wafer 105 rests against the polishing pad 102. The upper surface of the wafer 105 is against the lower surface of the carrier 106 of the arm 101. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 into the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense arm 107 extending across the radius of the polishing pad 102. The slurry dispense arm 107 dispenses a flow of slurry onto the polishing pad 102.
The slurry is a mixture of de ionized water and polishing agents designed to chemically aid the smooth and predictable planarization of the wafer. The rotating action of both the polishing pad 102 and the wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafer 105 at some nominal rate. This rate is referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from surface topography. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, degrading wafer through-put of the fabrication process. If the removal rate is too fast, the CMP planarization process will not be uniform across the surface of the wafers, degrading the yield of the fabrication process.
To aid in maintaining a stable removal rate, the CMP machine 100 includes a conditioner assembly 120. The conditioner assembly 120 includes a conditioner arm 108, which extends across the radius of the polishing pad 102. An end effector 109 is connected to the conditioner arm 108. The end effector 109 includes an abrasive conditioning disk 110 which is used to roughen the surface of the polishing pad 102. The conditioning disk 110 is rotated by the conditioner arm 108 and is translationally moved towards the center of the polishing pad and away from the center of the polishing pad 102, such that the conditioning disk 110 covers the radius of the polishing pad 102, thereby covering nearly the entire surface area of the polishing pad 102 as the polishing pad 102 rotates. A polishing pad having a roughened surface has an increased number of very small pits and gouges in its surface from the conditioner assembly 120 and therefore produces a faster removal rate via increased slurry transfer to the surface of the wafer and from more effective application of polishing down force. Without conditioning, the surface of polishing pad 102 is smoothed during the polishing process and removal rate decreases dramatically. The conditioner assembly 120 re-roughens the surface of the polishing pad 102, thereby improving the transport of slurry and improving the removal rate.
Referring still to FIG. 1A and FIG. 1B, the polishing action of the slurry determines the removal rate and removal rate uniformity, and thus, the effectiveness of the CMP process. As slurry is "consumed" in the polishing process, the transport of fresh slurry to the surface of the wafer 105 and the removal of polishing by-products away from the surface of the wafer 105 becomes very important in maintaining the removal rate. Slurry transport is facilitated by the texture of the surface of the polishing pad 102. This texture is comprised of both predefined pits and grooves 103 that are manufactured into the surface of the polishing pad 102 and the inherently rough surface of the material from which the polishing pad 102 is made.
To maintain the required degree of roughness in the surface of the polishing pad 102, the conditioner assembly 120 re-roughens the surface of the polishing pad 102 to counteract the smoothing effect of friction with the wafer 105. Without active conditioning by the conditioner assembly 120, the textured surface of the polishing pad 102 is quickly worn down and smoothed. The abrasive action of the slurry, the frictional contact with the wafer 105, and the frictional contact with the carrier ring 112, all combine to smooth away the needed texture of the surface of the polishing pad. Thus, the additional element, the conditioner assembly 120, is included on CMP machine 100, because without active conditioning, the surface of polishing pad 102 is smoothed and removal rate decreases dramatically.
Referring now to FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D, the relationship between the wafer, a carrier ring, and a polishing pad are shown (for teaching purposes, the above elements are not necessarily drawn to scale). FIG. 2A and FIG. 2B show a wafer 105 and a carrier ring 112 respectively. FIG. 2C and FIG. 2D show a side view of the wafer 105 in the carrier ring 112 on a polishing pad 102. As described above, the wafer 105 is held in place on the arm (not shown) by the carrier ring 112 as the polishing pad 102 rotates on the polishing platen. The carrier ring 112 accepts the wafer 105 within its inner radius surface 201. The upper surface of the wafer 105 is against the carrier 106 (not shown) of the arm. The carrier 106 (not shown) presses the wafer into the polishing pad with a predetermined force. As the polishing pad 102 rotates, carrier 106 (not shown) rotates the wafer 105.
Referring still to FIG. 2D, the wafer 105 typically protrudes slightly, relative to the lower surface of carrier ring 112. This gives the polishing pad 102 and the slurry (not shown) on the polishing pad 102 an even contact with wafer 105. The carrier ring 112 holds the wafer 105 in place while the polishing pad 102 and slurry polish the wafer 105. Polishing pad 102 frictionally slides against the lower surface of carrier ring 112 and against wafer 105. The predetermined amount of down force increases the friction between polishing pad 102, carrier ring 112, and wafer 105, thus, increasing the removal rate while at the same time increasing the rate at which the texture of the polishing pad is worn away and smoothed.
Thus, in CMP machines in accordance with the prior art, an additional element, the conditioner assembly 120, needs to be included, because without active conditioning, the surface of polishing pads used with the CMP machines are quickly smoothed. As described above, the conditioner assembly included with a prior art CMP machine is important to maintaining a stable removal rate. As such, the conditioner assembly needs to be carefully calibrated in order to obtain optimum CMP performance (e.g., the areas on the surface of the polishing pad which receive conditioning need to be aligned with the areas on the surface which frictionally contact the wafer). Additionally, the cost of the hardware involved in fabricating the conditioner assembly itself is substantial. If a method where devised which eliminates the need for a separate conditioner assembly included on the CMP machine, costs involved in setting up, calibrating, and maintaining fabrication lines using CMP machines would be lower.
Thus, what is desired is a system which improves the performance of a polishing pad in a CMP machine. What is further desired is a system which maintains a higher removal rate by conditioning the polishing pad in the CMP machine, yet is not burdened with the expense and maintenance requirements of a separate conditioner assembly. What is further desired is a system which ensures the areas on the surface of the polishing pad which receive conditioning are aligned with the areas on the surface which frictionally contact the wafer. The system should also be adapted to counter the added smoothing effects an additional amount of down force, applied to the upper surface of the wafer and carrier ring, has on the surface of the polishing pad. The present invention provides a solution to the above needs.